Variable reactance and control device comprising such a reactance



July-17 1951 J. L. ARENDS ETAL 2,561,041 f VARIABLE REACTANCE AND CONTROL DEVICE cou msmc sucu A REACTANCE Filed Oct. 22. 1948 3 Sheets-Sheet 1 Low rneoueucv v p AMPLIFIER Low PASS muss i mscammron STANDARD OSCILLATOR STAGE i I i 1 9 i l I 1i 1;) ,,W////;fiflxii,;;,;v i m 5s r i i v.2: I I u g L 1 Jn'cosus LUDOVICUS RRENDS AGENT r tented July 11, 1951 VARIABLE REACTANCE AND CONTROL DE- VICE COMPRISING SUCH A REACTANCE' Jacobus Ludovicus Arends and Eduard Herman Hugenholtz, Eindhoven, Netherlands, assignors to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application October 22, 1948, Serial No. 55,912 In the Netherlands December 13, 1947 11 Claims. 1

This invention relates to a variable reactance comprising a control member which is actuated by a control current and to control devices comprising such a variable reactance controlled by the control current. The invention may advantageously be used particularly in arrangements for automatic frequency correction (AFC).

For automatic frequency correction it is known to use an amplifying tube which may, for instance, be connected in parallel with the frequency-determining circuit of an oscillator and acts as a variable reactance. This permits extremely rapid AFC-control with a low control-power, high electrical sensitivity and low sensitivity to mechanical vibrations. However, the control range is small and such an AFC-arrangement exhibits a so-called set back force 1. e. that upon failure of the control voltage supplied to the reactance tube, the value of the apparent reactance formed by the tube is reduced to a given average value.

In other known AFC-arrangements these disadvantages are avoided. In these arrangements a motor starts in one or the other direction of rotation in accordance with the sign of the required frequency correction and thus actuates a control member, for instance in an intermediate electrode of a capacity to be controlled, in a direction corresponding to the direction of rotation of the motor. In this type of arrangements the motor is disengaged as soon as the required frequency correction has been effected, and in the absence of a control voltage or control current for some reason, the position of the control member does not alter, in other words there is no so-called set-back force.

AFC-arrangements of the last-mentioned type permit a comparatively great control range but, owing to the naturally involved mechanical inertia, rapid frequency-variations cannot be followed and the system is sensitive to mechanical vibrations. Moreover, the motor as compared with a reactance tube, requires a very high controlpower. In general, satisfactory electrical sensitivity is obtainable only with the use of powerful amplifiers.

AFC-arrangements comprising reactance tubes and motor-controlled reactances are often used simultaneously in connection with the complementary properties with respect to control speed, control sensitivity, control range, set-back force and sensitivity to mechanical vibrations.

The invention concerns inter alia a control device comprising a reactance which is variable by a movable control member and closely related with control devices comprising a motorcontrolled reactance, but which permits a satisfactory compromise, particularly for AFC-purposes to be arrived at between the properties stated in the previous paragraph.

According to the invention the control member consists of a magnetically actuated piston in a liquid-filled tube, and the parts of the variable reactance, which cooperate electrically with the control member, are structurally integral with the tube.

The arrangement according to the invention has a minimum of moving parts and does not comprise fast moving parts, whilst the force required for actuating the piston, and consequently the required control power, is comparatively low. Moreover, any troublesome mechanical initial friction can be avoided and there is no set-back force.

If the piston fits closely in the tube it can move only very slowly, since all of the liquid displaced must pass between the piston and the wall of the tube. The damping of motion thus introduced is variable within wide limits inter alia by the choice of the viscosity of the liquid. If desired, it may be increased by providing transverse grooves in the wall of the piston, by which grooves turbulence is brought about in the liquid. A judicious choice of the damping of motion is obtainable by correct aperiodic control (critical damping) at a comparatively high rate of control. Of course, in choosing the liquid used, the extreme ambient temperatures and the consequent variations of viscosity and internal friction should be taken into account.

If the average specific gravity of the piston corresponds to the specific gravity of the surrounding liquid the floating control member, in the event of mechanical vibrations of the tube, does not move with respect to the latter, so that if the parts of the variable reactance electrically co-operating with the control member, are structurally integral with the tube, external mechanical vibrations are practically unable to affect the value of the reactance. In the case of a floating piston there is no mechanical initial friction at all.

The movement of the control member may be utilised for varying the inductance of control coil wound on the tube, by causing the piston to influence the reluctance of the field of the coil (field displacement or field concentration by nonmagnetic and magnetic material respectively). In this event, however, the control coil must sometimes be magnetically decoupled with respectto the exciter coil(s) actuating the control 3 member, in order that in the event of controlcurrent variations disturbing currents induced by them in the control coil may be avoided.

This difiiculty is avoided with the use of a capacitative reactance to be controlled, when the piston, which may consist of metal. may be used as a capacitative intermediate electrode.

In order that the invention may be clearly understoodandreadilycarriedintoefiectitwili nowbeexplainedmorefuilybyreferencetothe acccmpanyingdrawingsgivenbywaycfeaample.

Fig. 1 shows a single side-band receiver of a typeknownperse,andasuitableformofan APO-arrangement according to the invention.

Fig. 2 is a perspective view of the component parts of a variable condenser according to the invention, and

Fig. 3 shows a single side-band receiver and a particularly suitable form of an AFC-arrangement according to the invention.

The embodiment of the invention shown in Fig. 1 comprises an antenna l for receiving a single side-band signal with suppressed carrierwave, which signal is supposed to comprhe the intelligence signals and in addition a to. pilot frequency represented by an incoming frequency of 500 he. The carrier-wave signal required to be added for detection of the incoming signals should have a frequency of 490 kc. and is derived from an automatically frequency-corrected local oscillator I. The incoming signals and the local oscillator signal are mixed in a mixer stage 8 for detection. 7 The detected signals appearing in the output circuit are divided in regard to frequency, by means of a low-pass filter l and a band-pass filter 5, into intelligence signals and a pilot frequency which are supplied to a low-frequency amplifier I connected to a reproducing device I and an amplifier I respectively.

The frequency of the carrier-wave signal producedby the local oscillator I is required to be automatically frequency-controlled by means of a reactance 9 which is connected in parallel with the frequency-determining oscillatory circuit in such manner that the pilot frequency corresponds exactly to the frequency of a comparison signal derived from a standard oscillator ll comprising a 10 kc. control crystal Ii.

To obtain the required control voltage the pilot signal and the comparison signal are fed to a discriminator I! known per se which supplies a direct voltage, of which the polarity and value.

in the event of frequency difi'erences and phase differences respectively, correspond to the polarity and value of the frequency diilerence and phase-difference respectively of the signals compared. The positive and negative control direct voltage is set up at the discriminator output-resistances It and I4 respectively and are fed, as the case may be through limiters II, to a balancing control stage iI, of which the output direct current, in accordance with the polarity of the control voltage, excites one or the other of two exciter coils ii. I. of the variable reactance I.

The variable reactance 9 comprises a hollow metal e. g. copper or aluminium piston which is movable in an axial direction in a liquidfilled glass tube it, and comprises a central annular soft iron core 2| For balancing the piston 20 mechanically with respect to the centre I2, which is of importance for insensitivity of the control member to mechanical vibrations. balancing nuts 28 are provided at both ends of the ccilslLIlandbeydndthemoveraconsiderable length of the piston. However, the capacity between the electrode II and the piston ll does practically not vary upon displacement of the pistonasindicatedbyarrcwsltsincethepiston ends constantly project sumciently from the electrode 2|.

Contrary thereto, the capacity between the electrode 24 and the piston II varies considerably upon displacement of the piston. since this electrode surrounds the left-hand end of the piston sothat the active surface varies considerably upon displacement of the piston. In an experimental construction of the device shown in Fig. 1, in which the variable capacity I corresponded, as regards construction and sine, to that shown in this figure, the following data were measured:

Speed of displacement with ampere-turns excitation: 0.5 mm./sec. Maximum piston stroke: 10 mm. Total detuning possibility: 40 kc. with 500 kc.

Speed of readiustment 2 hc./sec. with aperiodic control.

Minimum energization for readjustment: i0

ampere-turns.

Sensitivity: the piston moment stops and reverses by a number of ampere-turns caused by a detuning of a few kc./sec.

Mechanical sensitivity: vigorous tannins in an axialortransversedirectiononthetubell does not cause any detuning. Only with synchronism between pilot signal and comparison signal a phase variation. discernible by means of an oscillographically projected Lissajousfigure, occurs between the compared signals.

In the construction shown in Fig. l. the glass.

tube is constitutes the dielectric of the controlled capacity, and the capacity variation amounted to approximately 15 micro-microfarads at the most. The use of insulating material having a higher dielectric constant permits the capacity variation to be considerably increased.

Instead of the tube II the piston wall may, of course, act as a dielectric. In this event the intermediate electrode is provided on the inner wall of the piston and the electrodes 2|, 2! form parts of the wall of the tube ll.

If the piston body has a permanent magnetic core, it is sufiicient to use a single exciter coil instead of the coils l1 and II.

Fig. 2 shows a simple form a variable condenser according to the invention, in which the piston is constituted by a rotatable winged member.

This device exhibits inter alia the advantage that. unless torsional vibrations collie into play. the ratio between the average specific gravity of the winged member and the liquid may vary within wide limits without any troublesome effects. which is of great importance, for example with the use in airplanes. Satisfactory damping of. motion of the winged member can easily be obtained.

The variable condenser shown in Pig. 2 comprises a control member consisting of a rotatable winged member 21 which is placed in a liquid-filled glass tube 80 cemented in a circular groove 2! of a base it, and which is actuated by two exciter coils II, II surrounding the tube it. The winged member 21 consists of a soft iron strip curved into an S in such manner that its ends extend in a cylindrical plane around the axis of rotation of the winged member. The winged member 21 is rotated under the influence of the exciter coils ii. 32 constituting a rotaryfield exciter system. To this end, the said coils surround the wall of the tube in such manner that their pole shoes 33 and I are relatively displaced by 90 in space. The rotation of the winged member is limited in both directions by two bars 35 and it which are secured, at both sides of the shaft of the winged member, to the base It and carry another bearing plate II for the shaft of the winged member.

Apart from the means, referred to in connection with the construction of the variable reactance shown in Fig. 1, for rying the dampin; of motion of the control member, for instance a definite choice of the viscosity of the liquid, the damping of the winged member in the aforesaid construction is also determined by the width of the bars ll and It.

The winged member 21 constitutes a cameltative intermediate electrode and co-operates electrically with two coatings II and 3! cemented side by side on the outer wall of the tube II and partly surrounding the outer wall of the tube. These coatings may, for example, be connected to the ends of a frequency-determining circuit of an automatically frequency-corrected The rotation of the soft iron winged member caused by the action of the exciter coils II and I: may also be utilised for varying the inductance of a control coil which is constructed similarly to the exciter coils II, 32 and surrounds the wall of the tube.

If a high-frequency control coil is used. the yoke should consist of high-frequency magnetic material and rotation of the soft iron winged member will bring about a displacement of field and consequently a decrease in inductance of the control coil.

It is also possible to obtain an increase in inductance of the high-frequency control coil by securin to the winged member a core consistim of high-frequency magnetic material.

It. 3 shows a single side band receiver of a type known per se with a particular suitable form of an AFC-device according to the invention.

After amplification in a high-frequency amrlification sta e ll, the singl side band signal with suppressed carrier wave picked-up by an antenna ll, which signal is supposed to comps'lsc the intelligence signals and in addition a kcJsec. pilot frequency, is supplied to a first mixer stage II jointly with the oscillator slsnal fun a first crystal-controlled local oscillator 42.

6 The intermediate frequency ia al of. my. 800 kc./sec.-496 kc./sec. obtained upon mixing. in

which the frequency 'of 500 kc./sec. represents the 10 kc./sec. pilot frequency, is mixed, after intermediate frequency amplification (44), in a second mixer stage ll. with the frequency of 490 kc./sec. produced by an' automatically frequency-corrected local oscillator it.

The single side-band signal appearing in the output ircuit of the mixer stage 45, which signal extends over a frequency range of, say, 10-8 kc./sec. after having been transposed twice, is supplied. respectively, after further intermediate-frequency amplification (41), to a demodulator it through a separating stage It and to an amplifier Ii, preferably comprising a limiter, through a band-pass filter It for separating the intelligence signals and the pilot frequency.

To the demodulator 4! is supplied a local oscillator signal from a standard oscillator l2 comprising a 10 kc./sec. control-crystal it, and the low-frequency signals (0 to 4 kc./sec.) appearin; in the output circuit of the mixer stage 4! are supplied, through a low-pass filter It and a low-frequency amplifier it, to a reproducing device ll connected thereto.

The frequency of the signal produced by the local oscillator 46 is required to be automatically frequency-controlled in such manner that the pilot-frequency exactly corresponds to the frequency of the comparison signal from the standard oscillator 52.

To this end the pilot signal from the amplifier II is supplied to a band-pass filter discriminator 81 tuned to 10 kc./sec. and a comparing discriminator it (for example a mixer stage or a rotary field discriminator) to which the'comparison signal is also supplied. It is pointed out that a single discriminator may be sumcient.

The comparison discriminator it supplies a corresponding control voltage as regards polarity and value of the phase difl'erence between pilot-signal and comparison-signal, and serves for controlling a reactance tube 59 for rapidly correcting comparatively small frequency deviations which tube is coupled with the frequency- :laetermining oscillatory circuit of the oscillator The absolute discriminator 51 supplies a corresponding control voltage in regard to polarity and value of the frequency difference and serves for controlling a variable reactance 60 for (slow) correction of comparatively considerable frequency deviations, which reactance is likewise connected in parallel with the frequency-determining oscillatory circuit of the oscillator 6. Naturally, the discriminators 51, 58 should have relatively different ranges matched to the control ranges of the frequency-correctors 59, 60 to be controlled. The positive or negative control voltage of the discriminator I! is set up at the output resistances GI and 62 respectively and is supplied to a push-pull amplifier 63, of which the output direct current, in accordance with the polarity of the control voltage, energises one 1 or the other of two excitcr coils 64, 65 of a variable reactance it.

are substantially entirely filled with a neutral liquid, for instance kerosene, and are interconnected by a metal tube ll of which the diameter is smaller than that of the liquid-filled chambers. The resulting constriction in the metal vessel thus formed embraces the glass piston tube 68. The two exciter coils 64, 60 are provided in the constriction between the liquidfllled chambers. These coils preferably have an external diameter which is a little smaller than that of the liquid-filled chambers, which permits the coils easily to be enclosed in a tropic proof manner.

In addition to a structural shape suitable for manufacture-the advantage is obtained that no troublesome influence is experiencedfrom any air bubbles appearing in the liquid. Any air bubbles forming in the pistontube are removed, upon displacement of the piston, from the piston tube to the liquid-filled chambers and cannot regain access to the piston tube.

With the use of insulating material having a Starting from the central position of the piston 61 it will move to the left or to the right in accordance with one or the other of the coils 64 or 65 being excited. In order that the core 68 is surrounded by the two exciter coils even with maximum displacement of the piston from its central position, the core 68 has a width exceeding the stroke of the piston.

The piston constitutes a capacitative intermediate eiectrode and co-operates electrically with a cylindrical coating 12 cemented to one of the ends of the piston tube on the outer wall of the latter, and with the electrode formed by the wall of the tube II. These fixed electrodes II, I2 are respectively connected to the ends of the frequency-determining oscillatory circuit of the local oscillator 46, as shown diagrammatically by leads l3 and I4. Since the electrode constituted by the wall of the tube H extends over a considerable length of the piston, the capacity between the said electrode and the pis ton, upon displacement of the piston from its central position, does practically not vary, since the piston ends constantly project sufficiently from the tube. 1

However, the capacity between the coating 12 and the piston 51 varies considerably upon displacement of the piston, since this coating surrounds the left-hand end of the piston so that upon displacement of. the latter the active surface varies considerable.

In a test construction of the device shown in Fig. 3, in which the variable capacity 60 corresponded in regard to construction and value, to that shown in this figure, the following values were measured I Speed of displacement with 210 ampere-turns excitation: 1.7 mm./sec.

Maximum piston stroke 10 mms.

Total detuning possibility kc./sec. for 1000 kc./s.

Speed of readjustment: 1 kc./s. with aperiodic control. v Minimum excitation for readjustment; 12 ampere-turns. Sensitivity: the piston movement stops and reverses its direction by a number of amperetums caused by a detuning of a few cycles/sec. If the piston body comprises a permanent magnetic core 29 the sensitivity increases considerably.

In the example illustrated in Fig. 3 the glass tube 86 constitutes the dielectric of the controlled capacity and the capacity variation higher dielectric constant the capacity variation may again be increased materially.

If the piston body comprises a permanent magnetic core, a single excitor coil, instead of coils N, 65, is sufficient.

The movement'of the piston caused by th exciter coils 64, 68 may also be utilised for varying the inductance of a control coil wound on the glass tube, by causing the piston to influence the magnetic reluctance of the field of the coil (field displacement and field concentration by :lOII-IIIGS'DBtlC and magnetic material respective- To permit constantly an indication of the piston position, a further coating 15 is provided, similarly to the coating II, at the other end of the glasstube 60, and the capacity varying in accordance with the piston displacement, which capacity is formed between the coating I0 and the wall of the (earthed) vessel, is included in a Wheatstone bridge circuit, as shown diagrammatically by the condenser 16 indicated in dash lines.

To this bridge circuit a 10 kc./s. signal from the standard oscillator 52 is supplied, through a transformer 11, through the intermediary of a phase-shifting network 19. While the piston 61' occupies its central position the bridge circuit is set in such manner that no voltage appears in the output circuit. In this event a'phase difference between the voltages to be compared cannot occur and the pointer occupies its middle position. In accordance with the direction of movement of the piston, a voltage leading or lagging by 90 with respect to the comparisonsignal will be supplied to the phase-indicator through the bridge circuit, and the deviation of thepointer'with respect to its middle position will indicate the correct piston position.

01 course, it is also possible to use an optical device indicating the piston position.

Again it is possible to use a means, for example an electron ray tube, in the receiver, for indicating the piston position. If the comparison frequency and the pilot frequency are supplied to a tuning indicator constructed as an electron ray tube, the occurring beat-frequency is perceptible if the two frequencies are substantiali equal. 1 I

In the foregoing the invention has been set out with reference to particular forms of an AFC-device in single-sideband receivers. However, the invention may also be used in other forms of the AFC-device and both in transmitter and in receivers.

Furthermore the invention may be used in other control devices wherein variable reactances are used as control members, for example automatically controlled phase-shifters, for automatic tuning Wheatstone bridge circuits and the like.

What we claim is:

l. A voltage-responsive variable reactance de-' 'movable electrode within said tube with respect to said fixed electrodes, thereby to vary the ca- 9 pacitance between said fixed electrodes in accordance with an applied voltage.

2. A voltage-responsive variable reactance device comprising a liquid-filled tube of dielectric material, a movable electrode disposed within said tube and constituted by a metallic piston having a portion thereof of magnetic material, first and second fixed electrodes secured at spaced positions on the outer surface of said tube, and voltage-responsive electromagnetic means surrounding said tube for controlling the position of said piston within said tube in either direction with respect to said fixed electrodes, thereby to vary the capacitance between said fixed electrodes in accordance with an applied voltage.

3. A voltage-responsive variable reactance device comprising a liquid-filled glass tube, a metallic piston disposed within said tube, an annular magnetic member aflixed to said piston, first and second fixed electrodes secured at spaced positions on the outer surface of said tube, and an electromagnetic coil surrounding said tube for controlling the axial position of said piston within said tube.

4. A voltage-responsive variable reactance device comprising a liquid-filled tube, a movable electrode disposed within said tube and constituted by a hollow metallic piston having a portion thereof formed of magnetic material. first and second electrodes secured at spaced positions on the outer surface of said tube, an electromagnetic coil surrounding said tube to control the position of said piston within said tube with respect to said fixed electrodes, said piston having an average specific gravity corresponding to that of the liquid contained in said tube 5. A variable reactance device, as set forth in claim 4, further including an adjustable balancing member secured to one end of said piston.

6. A voltage-responsive variable reactance de-- vice comprising a liquid-filled cylindrical tube of dielectric material, a movable electrode disposed within said tube and constituted by a metallic piston having a portion thereof formed of magnetic material, said piston being axially movable within said tube, first and second annular electrodes disposed at longitudinally spaced positions to the outer surface of said tube. and an electromagnetic coil surrounding one of said annular electrodes f r controlling the position of said piston within said tube.

7. A voltage-responsive variable reactance device comprising a liquid-filled the capacitance between said tube. a movable sauce electrode disposed within said tube and constituted by a rotatable winged member of magnetic material, first and second fixed electrodes disposed at circumferentially spaced positions on the outer surface of said tube, and voltage-responsive electromagnetic means surrounding said tube for producing a rotating field rotating said member with respect to said fixed electrodes in accordance with an applied voltage, thereby to vary the capacitance between said fixed electrodes.

8. A variable reactance device, as set forth in claim 7, wherein said winged member is formed by a strip of magnetic material curved into the shape of an s such that its ends extend in an imaginary cylindrical surface concentric with the axis of rotation of the winged member.

9. A variable reactance device, as set forth in claim 8, wherein said electromagnetic means are constituted by a pair of exciter coils provided with pole shoes which are diametrically opposed with respect to the axis of the winged member.

10. A variable reactance device, as set forth in claim 9, further including a pair of damping bars positioned at either side of the axis of a winged member.

11. A variable reactance device comprising an open-ended cylindrical tube, a pair of liquidfilled chambers connected to either end of said tube, a metallic piston disposed within said tube and having a portion thereof formed of magnetic material, first and second electrodes secured at either end of said tube on the outer surface thereof, and an electromagnet surrounding said tube to vary the position of said piston and thereby electrodes. JACOBUS LUDOVICUS ARENDS. EDUARD HERMAN HUGENHOLTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,557,725 Proctor Oct. 20, 1925 1,641,438 Jones Sept. 6, 1927 1,875,329 Chireix Sept. 6. 1932 2,089,987 Urfer Aug. 17, 1937 2,332,044 Bell Oct. 19, 1943 2,456,463 Starie Dec. 14, 1948 I FOREIGN PATENTS Number Country Date Apr. 12, 1922 

